SAXS/WAXS/DSC studies on crystallization of a polystyrene- b-poly(ethylene oxide)- b-polystyrene triblock copolymer with lamellar morphology and low glass transition temperature

Abstract

Crystallization of a polystyrene- b-poly(ethylene oxide)- b-polystyrene (S–EO–S) triblock copolymer, S 40EO 136S 40, with lamellar morphology in the melt and low glass transition temperature ( T g=47 °C) of the S block was studied. The triblock copolymer was cooled from ordered melt and isothermal crystallization was conducted at crystallization temperatures ( T c) near the T g of the S block. It is found that crystallization behavior of S 40EO 136S 40 strongly depends on T c. When T c is far below T g, an Avrami exponent n=0.5 is observed, which is attributed to diffusion-controlled confined crystallization. As T c slightly increases, the Avrami exponent is 1.0, indicating that crystallization is confined and crystallization rate is determined by the rate of homogeneous nucleation. When T c is just below the T g of the S block, crystallization tends to become breakout and accordingly Avrami exponent changes from 1.0 to 3.2. Crystallinity and melting temperature of the EO block in breakout crystallization are slightly higher than those in confined crystallization. Time-resolved small and wide angle X-ray scattering (SAXS/WAXS) were used to monitor isothermal crystallization of S 40EO 136S 40. It shows that the long period is constant in confined crystallization, but it gradually increases during breakout crystallization. WAXS result reveals that confined or breakout crystallization has no effect on the crystal structure of the EO block.